Baseline Primary Drive System for the Explorer Class starship design.
Out of the likely ashes of my Multi-cycle Ram Augmented Interstellar Ramjet (RAIR) drive, I kept one solid piece. The idea of launching fuel to the ship from a fixed launcher in Sol (our home starsystem). The fuel/reaction mass mixtures could be launched as anything from aspirin sized pellets, to truck sized expendable mini-tankers. The former could be scooped up and fused as is, the later could be docked and off loaded.
I'm not going to go into a detailed analysis of the pros and cons of the different systems, but their are a lot of pros and cons for each. A stream of pellets would be easier to launch and offer less impact danger to the ship. But the pellets would be harder to keep together at a distance from the launcher, and harder for the ship to recover. The ship would need a ramscoop or something to scoop up its fuel, and when the packets drifted too far to the sides, the ship couldn't pick them up.
Mini-tankers on the other hand could use onboard rockets to maneuver to meet the ship. Possibly the ship could beam power to them (via lasers or microwaves) that could be used to drive the tankers attitude rockets, and order it to maneuver in front of the ship for pick up. Or a system like passive laser launchers could be used (which is discussed below). These would just require the tanker to have a large block of reaction mass on its backside. The ship could boost, and steer the tankers remotely using lasers; without any equipment on the tankers. Either way the ship could catch fuel tankers that had drifted thousands, possibly tens of thousands of miles off to the side of the ships flight path. Far more then possible with a magnetic scoop system.
Either way an externally feed system would mean that, the ship wouldn't need to carry the tremendous tonnage of fuel and reaction mass it would need for the flight. Launched acceleration fuel and reaction would only be carried by the ship from the time it entered the holding tanks, to the time it was burned. How this would work in each phase of the flight is described below
Assume the average fuel canister is the size of a 6 meter in diameter cylinder about 5 meters long. I think that should hold about a hundred tons of fusion fuel (6Li?) but of course that would vary with the type of fuel. This canister is heavily reinforced (you'll see why), and the ends are covered in a thick plug of reaction mass (could be anything from fiber reinforced ice, to solid Kevlar). A floating laser 'tug' fires a laser at this plug of reaction mass. One quick pulse to vaporize a layer off the bottom. Then a heavy pulse to turn the vapor to super heated plasma. I.E.. a pulsed rocket. Keep repeating these cycle up a couple hundred times a second, and you have a laser rocket. Specific impulse is limited by the type of reaction mass and the heat the laser brings it to.
Course corrections are handled in two ways. The first involves aiming the beam to one side of the base, rather than in the center. The uneven thrust will turn the canister, and subsequent pulses will thrust along the new vector. Precise course changes are handled by burning a bit of reaction mass off the side of a canister as it passes a laser tug. This would give precisely controlled lateral thrust.
Note the canister has no internal systems. Range is limited by the optical precision of the laser. Given what a Hubble telescope can do over interstellar ranges. I'll assume the system can aim acceptably out to 100,000 miles. So, if you station a laser tug every 100,000 miles or so. The tugs can take turns boosting a string of canisters. Given orbital mechanics. (No stable orbits that can keep you in a straight line.) They will have to be continuously boosting themselves around to stay acceptably close to the 'Launcher' track. (The exact position of the tugs isn't important, as long as they know exactly where they and the canisters are.)
Given this system the 'launcher' can be as long as the number of tugs, or as you need at the moment. If you space the tugs out every 60,000 miles (about 100,000 kilometers) for 100,000,000 kilometers (about a 1,000 tugs spaced from here to Mars.) The average G load on a canister exiting at a speed of 1/3rd the speed of light (100,000 kilometers per second), is 50,000 m/s^2, or about 5,000 G's. The canisters would need to take that thrust for the 2000 seconds it would take them to clear the acceleration track. Which seems acceptable for a solid block of reinforced metal and whatever.
The major problems with this system are the amount of reaction mass, and power, necessary to reach the desired maximum speeds may be prohibitive. So a laser sail may be necessary for the primary speed boost.
The ship uses a variation of the fuel launcher to catch the fuel canisters. The ship uses bow lasers that fire on the back of the canister to boost it forward and steer it onto the ships course vector. Assuming the laser booster can function hitting the base at over 60 degrees off the canisters axis, and can hit the target at 100,000 km. The ship can 'catch' (I.e. Steer to itself) a fuel canister over 80,000 km off to the side. Far better than the scoop systems we were considering. Which could increase the range at which the ship can be externally fueled. (Hopefully out to about 3,000 au's, since it would take that far for the ship to boost to 1/3rd c.)
If we assume a fusion fuel with a specific impulse of 2 million (we might even get to 2,400,000 if we push it) the ship would need to receive 5 times its weight in fuel to boost it to 1/3rd of light speed (100,000,000 m/s). If the canister and its reaction mass is added into the ships budget it might be able to use less fusion fuel (thou more total mass) to get to same speed.
The ship would be heavy. Not only with the weight of its own systems and structure, but food and equipment for the mission, exploration equipment, the crew and their homes; and weighing dozens of times all the rest combined, deceleration fuel (See fusion rocket mass tables). The ships maximum speed is limited by deceleration (braking) fuel it can carry. Given the years of flight time involved, every effort will be made to load the ship with fuel.
Pulling out of Our star system, Sol, the ship will accelerate at one ship G, accelerating 10 meters per second, per second. An orbiting set of laser tugs will boost the fuel and extra reaction mass the ships motors will need for this acceleration run. The ship will be continuously picking up these fuel packets, and feeding their contents into the fusion motors. This will continue until the ship reaches the maximum speed it can slow down from. If the orbiting launcher can still get fuel within pickup range of the ship. Extra fuel can still be sent out to increase the breaking supply. Possibly enough to extra fuel to allow the ship to accelerate a little more, and cut down the long flight.
Because of the deceleration fuel limitation, it is unlikely that the ship can get to more than a quarter or a third of the speed of light. But that's still a 100,000 kilometers per second. The ship will need to protect itself against impacts. One of the simplest ideas is to push several miles of charged dust ahead of the ship. Ramming a cloud of charged iron dust at 360,000,000 kilometers per hours will turn most anything into ionized plasma. Which can be shoved ahead of the ship, or off to the sides, by the charges dust cloud handler. Effectively most anything you run into at speed will become more shielding dust.
Now for the bad news - you have to slow down. We can't pre-load a deceleration course track into the target star with fuel across interstellar distances. So your stuck with the fuel you brought along. Unless we can come up with a neat magnetic trick to brake the ship in empty space (sorry no luck), its fire up the reactors and put engines into reverse. As the tables in fusion rocket shows, even slowing down from 1/3rd light speed would force the ship to carry 50 - 100 times its weight in fuel. See internally fueled fusion rockets for more detail.
Basically, for a fusion rocket with a specific impulse of 1,000,000. To decelerate a ship down from 1/6th the speed of light. The ship would need to carry 147 times its dry weight in fuel and reaction mass. If you can get a fusion rocket with a specific impulse of 2,500,000, it could decelerate you from 1/3rd the speed of light, with only 55 times the ships dry weight in fuel.
|Speed 50,000,000 m/s (1/6 light speed)||Speed 100,000,000 m/s (1/3 light speed)|
|7 to 1 mass ratio.||55 to 1 mass ratio.|
|12 to 1 mass ratio.||148 to 1 mass ratio.|
|27 to 1 mass ratio.||785 to 1 mass ratio.|
|147 to 1 mass ratio.||22,000 to 1 mass ratio.|
|22,000 to 1 mass ratio.||500,000,000 to 1 mass ratio.|
Note: a specific impulse of 1,000,000 (A exhaust velocity of 10,000,000m/s) means that the engine gives 1,000,000 pounds of thrust, for one second, for every pound of fuel consumed. This has long been a standard fusion engine performance number. (For comparison the best chemical engines have a specific impulse of 455.) Current designs might exceed 1,500,000. (See write ups on Bussards Fusion reactor, and Bussards Plasma rocket) In theory a specific impulse of 2,000,000 or more,might be possible.
At low speed interplanetary runs, the drive works like a conventional fusion rocket burning stored fuel and reaction mass. But the engines that could barely get us across interstellar space. Can now make this huge ship commute around the confines of a starsystem with ease. A ten hour burn of the main engines will would get you from Earth to Jupiter in about a month, or Mars in a week. A two day burn would get you to Jupiter in less than a week. Under four days of constant burn will get you to Mars. About a week of constant burn will get you to Jupiter.
Once in system the main ship will be shuttling surface teams, support ships, and equipment around the star system. It size and speed will allow it to drag around tremendous loads of equipment or raw material. It could haul ore in to a construction site for a space colony. Or to a fuel ore processing facility. (Note the crews will need to find enough fuel to get the ship out of the starsystem.) The starship will be carrying all the exploration equipment and personnel to anywhere of interest in the starsystem.
As the exploration phase comes to an end the support crew will be processing the tremendous tonnage of fuel ore necessary to refuel the ship for the boost to home. This fuel could be carried on the ship. All those fuel tanks you emptied decelerating into the starsystem could be filled to accelerate you back to home. (Assuming you can find that much fuel!) Or (possibly) the crew could construct a fuel launcher system like the one the ship used to leave home.
If an automated fuel launcher could be constructed in the target starsystem, and the one near earth relied upon for braking fuel, the ship could launch with her fuel tanks nearly empty. Without the massive load of fuel and exploration equipment the ship could weight a hundredth of its loaded weight when it left Sol. This would allow it to boost faster and to higher speeds, even if the local fuel launcher could only launch (and the local mines only supply) a tiny fraction of the fuel the Sol launcher could, launched to a shorter range than the sol Launcher could. With such a lightly loaded ship, most of the drive systems would be unnecessary. Allowing either further weight reduction of the ship, or allowing tremendous redundancy in the equipment.
A automated launcher system would not only give this missions crew a much shorter flight home. It could allowing future missions into this star system a much shorter round trip. However an automated fuel launcher may be too complicated for the crew to construct. It would require the construction of a thousand laser tugs and tens of thousands of the filled fuel canisters. But if enough construction gear can be brought along (or some ultra-tech like self replicating machines is assumed), this would greatly expand the amount of flights and exploration we could do to this star system.
Since the ship can assume that the orbiting fuel launcher in Sol will be turned on to help it slow down. The ship wouldn't need to be heavily loaded down with deceleration fuel. By this point in the mission the ship will be comparatively light. Its fuel tanks empty. Most of the food and consumables consumed. Probably most of the exploration gear left behind. The ship will be coming home needing fuel. Or will it?
The ships cruise speed will be a fraction of the speed of light, and it will be flying straight toward the fuel launcher. Obviously a nice neat docking with the incoming fuel packets is out of the question. You could of course use the scoop lasers to steer a mini-tanker in front of the ship and then explode the tanker. The contents will slam into the ships forward dust shield and be heated to plasma. That plasma could be scooped up and channeled into reactors to power the reversed engines. Or Is that necessary?
Whatever blasts through the dust shield is going to be hot! The forward electromagnetic barriers that shove the dust ahead of the ship are going to be ramming into, and pushing forward, a plasma ball an eighth of a mile in diameter and unknown length, ahead of the ship. If their is any fusion fuel in the mess, its probably going to be fusing out ahead of the ship. Effectively the front of the ship will be an open fusion motor. If the launcher isn't firing fuel. Whatever it is launching is still going to be a hellish fireball ahead of the ship. Effectively the ship will be breaking against this artificially induced drag source. Converting its kinetic energy into heat in the plasma.
You wouldn't even need to run the reactors to keep the electromagnetic nose shield charged. Ducting a little of the plasma through a central core will allow you to use the core as a generator. Converting the energy of the high speed plasma into electricity. Or more correctly converting the ships kinetic energy past the plasma stream into electricity. If you want to get tricky a magnetic 'wiggler' in the plasma stream could be used to convert some of the energy into a forward laser. The forward blast from a plasma laser charged by the kinetic energy of a few hundred thousand tons of charging starship should clear anything out of your path. It certainly will show everyone the explorers have returned home.